If climate change continues on its current course, the number of heat-related deaths will rise as temperatures climb during the summer, according to a report from the Natural Resources Defense Council (NRDC), an environmental advocacy group.

The organization projected on the basis of peer-reviewed data that global warming — expected to increase average temperatures in North America by 4º to 11ºF by 2099 — will be responsible for an extra 150,000 deaths tied to excessive heat events in the 40 largest U.S. cities by the end of this century.

The hardest hit, in terms of excess heat-related deaths by the end of 2099, are estimated to be:

Louisville, Ky. (18,988)

Detroit (17,877)

Cleveland (16,625)

Memphis, Tenn. (10,154)

Jacksonville, Fla. (8,141)

The estimates were based on two studies by Larry Kalkstein, PhD, a professor of geography and regional studies at the University of Miami in Florida, and colleagues, published in the last few years in Weather, Climate, and Society and Natural Hazards.

The researchers studied two baseline time periods — 1975 to 1995 and 1975 to 2004. They included 1996 to 2004 to account for the greater use of preventive measures against heat-related deaths during those years. When making projections into the future, they did not make adjustments for the growth or aging of the population in order to keep the estimates conservative, Kalkstein said on a conference call with reporters.

The NRDC report based on the data from the two studies focused on two variables — excessive heat event days and death attributable to extreme heat.

“Excessive heat event days occur when a location’s temperature, dew point temperature, cloud cover, wind speed, and surface atmospheric pressure throughout the day combine to cause or contribute to heat-related deaths in that location,” according to the report.

Aside from temperature, there are several factors that contribute to the number of excessive heat event days, including geography, urban structure, green space, the use of local warning and preventive measures, and the resiliency of the local residents.

Most vulnerable during these heat waves are seniors and young children, particularly those living in cities with a variable climate. Cities with relatively hot temperatures interspersed by spikes of extended extreme heat will see more health-related deaths than cities that are consistently hot, like Miami, Kalkstein explained.

According to the estimates from Kalkstein’s data, the average number of excessive heat event days per summer across all 40 cities was 233 from 1975 to 1995. By 2099, that was projected to increase to 1,918 per summer, assuming a continued reliance on fossil fuels and no significant policy interventions.

Accompanying that increase is a rise in the average number of heat-related deaths per summer from 1,332 from 1975 to 2004 to 4,608 per summer by 2099. All but three of the 40 cities would see more deaths by the end of the century.

Adding up all of the excess heat-related deaths would yield an extra 150,322 heat-related deaths attributable to climate change.

Kalkstein noted that there are measures that cities can take to mitigate the threat of extreme heat, including opening air-conditioned shelters when the National Weather Service calls an excessive heat warning; having a special hotline that people can call to get advice on dealing with heat-related illness; and increasing the number of staff in emergency rooms during heat waves.

He singled out Philadelphia as being particularly proactive in dealing with excessive heat events, noting that the city has volunteer block captains who go door to door checking on vulnerable individuals during heat waves.